The accuracy of net oil measurement is extremely important to buyers and sellers of oil. If the oil contains water, the buyer does not want to pay for the oil on the basis of the gross amount of liquid shipped to him. Rather, he wants to pay only for the net amount of oil present in the total volume delivered. Net oil measurement is also required in oil fields for royalty payments and in enhanced oil recovery fields for pumping rate control.
There are in the art a number of instruments which have been used to measure water content in an oil/water mixture. Most of such instruments utilize the dielectric properties of the fluids, e.g., the difference between the dielectric constant of water and the dielectric constant of oil. As such, the main problem with such devices is their inability to operate with respect to mixtures where the water constituent of the mixture is in the continuous phase rather than the oil phase. By definition, the dielectric constant is the ratio of the capacitance of a capacitor field with a given dielectric to that of the same capacitor having only a vacuum as the dielectric. Therefore, in using the devices for oil/water measurement, when water is the continuous phase, the instrument will show a maximum value because the electric path between the two parallel plates of the capacitor will be shorted by the water in continuous phase. Unfortunately, this phenomena exists even though oil may still comprise a high percentage (e.g., some 40 or 50% or more) of the overall mixture.
Of the prior known devices, U.S. Pat. No. 4,774,680 to Agar is related to the present invention. The Agar patent addresses the problem of single curve characteristics. Agar solves the prior problem of determining a fundamental measurement based upon an electrical property of the fluid. Corrections to the fundamental measurement are not taught or suggested by the Agar patent.
A relatively typical capacitance probe for use in determining oil/water ratios is found in U.S. Pat. No. 3,200,312 to Callaghan. Callaghan relies on the measurement of the mixture's dielectric constant. As such, the probe must be non-functional when conductive water is in the continuous phase.
Yet another capacitance-type probe is taught in U.S. Pat. No. 3,025,464 to Bond probe is designed specifically for pipeline use where there is typically low water content and oil is in the continuous phase. For that purpose, the Bond probe will function adequately. However, because the Bond probe is a capacitance probe, it will not function in mixtures where water becomes the continuous phase.
Still another prior art capacitance probe is shown in U.S. Pat. No. 3,523,245 to Love et al. It has the same shortcomings as the prior art references mentioned above. In fact, FIG. 2 of the Love et al. patent depicts a graph for water fraction versus probe capacitance. It is noted that the water fraction portion of the graph does not go above 0.5. In fact, the Love et al reference in discussing FIG. 2 specifally states that when the water fractions get above 0.5, the water tends to separate out and the capacitance quickly approaches the value at free water.
U.S. Pat. No. 3,368,147 to Graham teaches a capacitance measuring circuit to determine the sediment and water content of oil well production. Because Graham relies on capacitance, such reference is also insufficient to determine oil/water rations where water is in the continuous phase.
U.S. Pat. No. 3,550,019 to Thomas seems to teach a linearizing circuit for net oil analyzers. However, Thomas does not teach the use of a digital linearizer or any means of overcoming the jump in the electrical characteristic of the mixture as the oil/water mixture moves from oil being in the continuous phase to water being in the continuous phase.
Yet another net oil computer is described in U.S. Pat. No. 3,385,108 to Rosso. Rosso relies on a capacitance probe and does not teach the use of a digital linearizer or any means of overcoming the jump in the electrical characteristic of the mixture when the oil/water mixture goes from oil being in the continuous phase to water being in the continuous phase.
Another typical capacitance probe having the same inadequacies as those probes mentioned above is found in U.S. Pat. No. 3,006,189 to Warren et al.
A few techniques are available to measure the electrical properties of the mixture. For example, the conductivity of the mixture may be measured at a high frequency. While such techniques avoid the saturation effect which is typical of measuring capacitance, they produce two distinct, non-linear curves or families of curves of output signal. With the two distinct non-linear curves, the conductivity curves, the current may be plotted against the percentage of water in the mixture. The conductivity curves may be empirically or theoretically derived. The first set of these curves is for the case where the water is in the continuous phase, while the second set of these curves is for the case where oil is in the continuous phase. It should be understood that the "step jump" between these curves does not occur at a predetermined oil/water ratio. With respect to the "step jump" between the curves, other variables are involved including (as examples) the surface tension of the fluids and the amount of emulsifying chemicals present.
It is therefore a feature of the present invention to provide an apparatus and method for determining the percentage of water present in a mixture where either oil or water is in the continuous phase.
Yet another feature of the present invention is to provide an apparatus and method for determining the percentage of water in an oil/water mixture where the amount of water present is in the range from 0 to 100 percent.
Still a further feature of the present invention is to provide a method and apparatus for determining the water content in oil/water mixtures by measurement of the mixture's electrical properties wherein the output signal is linearized.
Still another feature of the present invention is to teach a method and apparatus for determining whether oil or water is in the continuous phase by measuring an electrical property of the mixture.
Yet a further feature of the present invention is to provide a method and apparatus for selecting one of two curves which represent the possible conditions of the media.
It is a further feature of the present invention to provide an apparatus and method for determining the percentages of oil and water in an oil/water mixture by measuring its electrical properties wherein it is determined whether the oil or the water is in the continuous phase.
Yet another feature of the present invention is to provide two empirically or theoretically derived curves which plot percentage of water versus current which allows determination of whether the oil or the water is in the continuous phase.
Briefly state, the foregoing and numerous other features, object and advantages of the present invention will become readily apparent upon reading the detailed description, claims and drawings set forth hereinafter. These features, objects and advantages are accomplished by utilizing a microprocessor or comparator circuit which is able to distinguish between oil being in the continuous phase and water being in the continuous phase based on the electrical properties of the mixture. For example, the conductivity of the media may be measured at a high frequency. While these techniques avoid the saturation effect which is typical of measuring capacitance, they produce two distinct, non-linear curves of output signal typically plotting current versus the percentage of water in the mixture. These curves may be empirically or theoretically derived. The first of these curves is for the case where the water is in the continuous phase, while the second curve occurs where oil is the continuous phase. It should be understood that the change in phase does not occur at a predetermined oil/water ration. Other variables are involved including droplet size, surface tension and emulsifying chemicals present. Typically however, the change occurs when the amount of water present in the mixture is in the range of approximately 35 to 75 percent of the total. Thus, just measuring the properties of the mixture does not give the complete solution. Because there are two distinct families of curves or equations it is necessary to determine which curve or equation is to be used in calculating the percentage of water present.